The present invention relates to processes for the production of elemental sulfur from an acid gas containing hydrogen sulfide.
Claus technology is widely used to recover sulfur from hydrogen sulfide-containing sour gas feed stocks such as off gases produced in natural gas processing and petroleum refining operations.
Typically, the sour gas feed stock is treated to concentrate the hydrogen sulfide content to about 20 mole percent or more in an acid gas feed stream that is then directed to the sulfur recovery unit. In a conventional Claus plant, part of the hydrogen sulfide content of the acid gas feed stream is combusted (i.e., oxidized to sulfur dioxide) in a reaction furnace supplied with air according to equation (1).
H2S+3/2O2xe2x95x90xe2x95x90xe2x95x90 greater than SO2+H2Oxe2x80x83xe2x80x83(1)
Due to the inert load of the combustion air, the volumetric flow to the reaction furnace could be twice that of the acid gas stream and the equipment of the sulfur recovery unit must be sized to accommodate the increased flow. The amount of oxygen introduced into the reaction furnace is carefully controlled in order to combust approximately one-third of the hydrogen sulfide content of the acid gas and provide a combustion gas containing one mole of sulfur dioxide for every two moles of hydrogen sulfide in accordance with the well-known Claus equation (2). Careful control of combustion gas stoichiometry maximizes conversion to sulfur.
2 H2S+SO2 less than xe2x95x90xe2x95x90xe2x95x90 greater than 3/N SN+2 H2Oxe2x80x83xe2x80x83(2)
A part (e.g., 60-70%) of the hydrogen sulfide and sulfur dioxide content of the combustion gas reacts in the furnace under combustion conditions to form sulfur and water. The gas exiting the reaction furnace then enters a waste heat boiler where some of the energy from the exothermic oxidation of hydrogen sulfide is recovered. A sulfur condenser is placed following the waste heat boiler to remove sulfur produced in the reaction furnace and lower the dew point of the cooled process gas stream. This gas is then fed to a catalytic stage containing a Claus conversion catalyst (e.g., activated alumina, bauxite or titanium dioxide) for promoting the reaction between hydrogen sulfide and sulfur dioxide. Prior to entering the catalytic stage, the gas is typically reheated in order to ensure that sulfur does not condense and deactivate the catalyst. In the catalytic stage, the Claus reaction takes place again to form additional sulfur and water, this time at a temperature considerably lower than in the reaction furnace. Product sulfur is removed in a sulfur condenser downstream of the catalytic stage. Since the Claus reaction is equilibrium controlled and higher conversion is favored by lower temperatures, a plurality of reheater, catalytic stage and condenser combinations (typically three) are employed in series to improve the overall sulfur recovery.
The tail gas exiting the condenser following the last catalytic stage still contains appreciable concentrations of sulfur-bearing compounds including unreacted hydrogen sulfide and sulfur dioxide, carbon-sulfur species such as carbon disulfide and carbonyl sulfide formed when hydrocarbons are present in the acid gas as well as sulfur vapor. Thus, in order to comply with emission standards, it is necessary in most cases to treat the tail gas in some fashion to reduce the concentration of these sulfur species prior to discharging the tail gas to the atmosphere. A hydrogenation system with a reducing gas generator and catalytic bed containing a cobalt-molybdenum catalyst may be employed in tail gas treatment. The hydrogenation system reduces both sulfur dioxide and sulfur vapor to hydrogen sulfide while hydrolyzing carbon disulfide and carbonyl sulfide to hydrogen sulfide. The hydrogen sulfide stream is then concentrated, typically using an amine absorbent process, and then recycled to the inlet of the sulfur recovery unit.
Although conventional Claus installations have served the sulfur industry well for many years, such installations can be very costly, both in terms of the initial capital outlay and ongoing operating expense. With increasing sulfur content of crude oil and natural gas, both petroleum refiners and natural gas processors are pushed for acid gas processing capacity. As known reserves are depleted, the sulfur content of natural gas and crude oil is likely to continue to increase as less attractive reserves are exploited. At the same time, ever-tightening environmental regulations demand lower and lower sulfur emissions. These forces are causing an increasing interest in new approaches capable of achieving high sulfur recoveries and increased process capacity within capital and operating cost constraints.
Proposed solutions intended to decrease the size and increase the capacity of a Claus installation include combusting the tail gas to oxidize the sulfur species present to sulfur dioxide, recovering a concentrated stream of sulfur dioxide from the combusted tail gas and recycling the concentrated sulfur dioxide to a point upstream of the sulfur recovery unit. In this fashion, it is possible to substantially avoid the inert load accompanying air used to oxidize hydrogen sulfide to sulfur dioxide and thereby decrease the size of sulfur recovery process equipment and/or increase plant capacity. In such a system, the complexity and expense associated with precise, constant control of the ratio of hydrogen sulfide to sulfur dioxide to optimize the Claus reaction may be eliminated. Moreover, it has been suggested to replace the Claus reaction furnace and multiple catalytic stages with a single catalytic stage combined with a tail gas treatment system to recover and recycle a concentrated sulfur dioxide stream to the catalytic converter. Such a sulfur recovery unit is disclosed in U.S. Pat. No. 5,628,977 (Heisel et al.).
Recycle of sulfur dioxide from tail gas treatment to a catalytic only sulfur recovery unit has the potential to reduce capital requirements and increase process capacity. However, such potential has not been fully realized. The high temperatures accompanying operation of a catalytic converter in reacting concentrated streams (e.g., as high as 90 mole percent or more) of hydrogen sulfide and recycle sulfur dioxide would result in significant equipment corrosion problems, rapid loss of catalytic activity due to hydrothermal aging of the Claus conversion catalyst and reduced sulfur formation. Thus, in the above-referenced patent, Heisel et al. teach that an installation including a single catalytic stage and recycle of concentrated sulfur dioxide from tail gas treatment can be used to process xe2x80x9crelatively cleanerxe2x80x9d acid gas streams containing no more than 20 percent by volume hydrogen sulfide. This restriction places a considerable limitation on the flexibility and capacity of the system.
Furthermore, regardless of configuration, Claus installations are susceptible to catalyst fouling and deactivation resulting from hydrocarbons present in the acid gas feed stream. Even in a conventional Claus installation, hydrocarbons in the acid gas may not be burned in the reaction furnace and pass through to the downstream catalytic stages. Catalytic cracking of heavier hydrocarbons such as n-octane and aromatics can lead to soot (i.e., elemental carbon) deposits on the Claus conversion catalyst. Carbon-sulfur species can also lead to soot formation. The coked catalyst may exhibit reduced activity and increased pressure drop. Unsaturated hydrocarbons including linear and branched olefins (e.g., alkenes) and aromatics such as benzene, toluene, ethylbenzene and xylene, sometimes referred to as BTEX, are particularly troublesome since they may polymerize and form gummy deposits that ultimately block the pores of the catalyst. The concerns regarding catalyst fouling from hydrocarbons in the acid gas feed stream are especially present in a catalytic only sulfur recovery unit wherein the acid gas is not exposed to the combustion conditions of a reaction furnace. The cost associated with frequent catalyst replacement or regeneration can add significantly to the operating costs of a Claus installation.
An activated carbon system has been reported for use in removing aromatics and heavier hydrocarbons from acid gas streams fed to a Claus installation. L. G. Harruff et al., xe2x80x9cActivated Carbon Passes Test for Acid Gas Cleanupxe2x80x9d, Oil and Gas Journal, Jun. 24, 1996, pp. 31-37. Similarly, hydrophobic polymeric resin systems can be used to remove unwanted hydrocarbons from the acid gas. However, both of these systems suffer from the fact that they remove hydrocarbons on the basis of vapor pressure rather than reactivity. Thus, lower molecular weight olefins such as ethylene or propylene would tend to pass through both processes virtually unchecked.
Therefore, a need remains for further improvements in existing Claus practices and solutions to problems faced by the sulfur industry.
Among the objects of the present invention, therefore, are the provision of an improved process for the production of sulfur from hydrogen sulfide contained in an acid gas feed stream by the Claus reaction; the provision of such a process including a catalytic only sulfur recovery unit coupled with recycle of sulfur dioxide-enriched gas from tail gas treatment capable of processing highly concentrated hydrogen sulfide-containing acid gas streams and providing improved process flexibility and capacity; the provision of such a process having a high sulfur recovery efficiency and low sulfur emissions; the provision of such a process that can be installed and operated at lower cost than conventional Claus installations having a reaction furnace followed by a plurality of catalytic stages and a tail gas treatment system; and the provision of an improved process for the production of sulfur from hydrogen sulfide including a catalytic stage wherein the problems caused by hydrocarbons in the hydrogen sulfide-containing acid gas are alleviated in a cost-effective manner.
Briefly, therefore, the present invention is directed to a process for the production of elemental sulfur from an acid gas feed stream containing hydrogen sulfide. A feed gas mixture comprising the acid gas feed stream and sulfur dioxide is contacted with a Claus conversion catalyst in a single Claus catalytic reaction zone at a temperature effective for the reaction between hydrogen sulfide and sulfur dioxide to form a product gas effluent comprising elemental sulfur and water. The product gas effluent is cooled to condense and separate elemental sulfur from the product gas effluent and form a tail gas effluent. A portion of the tail gas effluent is combusted with a source of oxygen in a combustion zone to oxidize sulfur species present in the tail gas effluent and form a combustion gas effluent comprising sulfur dioxide. The combustion gas effluent is contacted with a liquid absorbent for sulfur dioxide in a sulfur dioxide absorption zone to selectively transfer sulfur dioxide from the combustion gas effluent to the absorbent and produce an exhaust gas from which sulfur dioxide has been substantially removed and a sulfur dioxide-rich absorbent. Sulfur dioxide is then stripped from the rich absorbent in a sulfur dioxide stripping zone to produce a lean absorbent and a sulfur dioxide-enriched stripper gas. The lean absorbent is recycled to the sulfur dioxide absorption zone for further selective absorption of sulfur dioxide from the combustion gas effluent. The sulfur dioxide-enriched stripper gas is mixed with the acid gas feed stream and the remainder of the tail gas effluent to form the feed gas mixture introduced into the Claus catalytic reaction zone. The proportion of the tail gas effluent introduced into the Claus catalytic reaction zone as part of the feed gas mixture is sufficient to moderate the temperature within the Claus catalytic reaction zone.
The invention is further directed to a process for the production of elemental sulfur from an acid gas feed stream containing hydrogen sulfide and an unsaturated hydrocarbon component selected from the group consisting of linear olefins, branched olefins, aromatic hydrocarbons and mixtures thereof. Hydrogen sulfide from the acid gas feed stream is oxidized to elemental sulfur in a catalytic reaction zone containing an oxidation catalyst and supplied with an oxidant gas. In accordance with the present invention, the acid gas feed stream is pretreated upstream of the catalytic reaction zone to reduce the concentration of the unsaturated hydrocarbon component and thereby inhibit deactivation of the oxidation catalyst. The pretreatment of the acid gas feed stream comprises contacting at least a portion of the acid gas feed stream with an aqueous acid wash to react unsaturated hydrocarbons with the acid and form an addition reaction product. Thereafter, the addition reaction product is separated from the acid gas feed stream.
Other objects and features of this invention will be in part apparent and in part pointed out hereinafter.